Conveyor control systems have included self-propelled vehicles which have a motor thereon to rotate wheels on the vehicle to move the vehicle along a guide rail. The motor receives power from power rails running along the guide rail. Each vehicle is self-propelled so that if a vehicle breaks down, the vehicle can be removed without a total shut down of the conveyor system.
The following patents issued to the same inventor of this invention, relate to such a conveyor system. U.S. Pat. No. 4,984,521,issued Jan. 15, 1991 entitled Conveyor Control, and U. S. Pat. No. 4,892,980, issued Jan. 9, 1990 entitled Dual Contacts On Voltage Rail, and U.S. Pat. No. 4,924,164, issued May 8, 1990 entitled Software Zoning of Conveyor Control, and U.S. Pat. No. 4,919,057, issued Apr. 24, 1990 entitled Conveyor Control Through Binary Coding. Each of these patents relate to a conveyor control system which has self-propelled vehicles with wheels, each driven by a motor to move the vehicle along guide rails. The system includes power rails for supplying power to the vehicle and may have at least one command rail which transmits a command signal indicative of a requested speed. The command signal may be in the form of an analog signal or a digital signal. Each vehicle includes control circuitry which interrupts the command signal to drive the vehicle at the requested speed. Other circuitry may be included to slow or stop the vehicle due to other vehicles in its path or faults in the circuit.
Such conveyor systems commonly use contact brushes which run along the power rails to receive power and transmit same to the vehicle. Such contacts at many times may periodically lose source power or lose neutral power. Industrial control standards call for a master control relay to provide safety to the control system and particularly to the vehicle in case of failure. Brief interruptions of the power or neutral cause the master control relay to deenergize, resulting in frequent nuisance of power downs. As a result of such power downs, many conveyor manufactures do not have a master control relay included on the on-board vehicle controls. An undesirable effect is that the whole vehicle can become electrically hot when loss of neutral occurs, leading to injury or electrocution. Another safety problem is that a failure in the drive electronics can result in a runaway vehicle.